Bulk polymerization processes are employed to prepare copolymers of rubber and plastic forming monomers, however, they require a second stage shearing step or the particle size of the rubber formed is very large and non-uniform. Emulsion processes produce a more favorable particle size and better gloss product but have the disadvantage that water is included in the process.
One way in which particle size of the rubber can be controlled without resort to an emulsion system is by employing a reactor extruder as the polymerization vessel. Reactor extruders of the twin screw design provide sufficient shearing action during polymerization of the monomers within that smaller rubber particle sizes can result.
The continuous production of copolymers from 1,3-dienes and monovinyl aromatics in an extruder is described in U.S. Pat. No. 3,780,139. The patent does not disclose, however, the formation of resin copolymers of monomers such as styrene, acrylonitrile, methylacrylate and the like grafted onto a preformed rubber.
Work with preformed rubbers in the presence of various plastic monomers which are polymerized in a reactor extruder has been set forth in a copending patent application, U.S. Ser. No. 309,718, now U.S. Pat. No. 4,410,659 commonly owned by the Assignee of record herein. A feature of that application includes dissolving a rubber in the plastic monomers prior to feeding into the extruder, coupled with the shearing action within the extruder to produce a uniform dispersion of the rubber throughout the plastic phase yielding high impact strength materials.
A process which also discloses dissolving a preformed rubber in a solution of one or more plastic forming monomers is set forth in U.S. Pat. No. 3,511,895. The patent calls for a three stage mass polymerization conducted in separate reactors having agitators for shear and mixing. The resulting polymer product has a broad and uniform distribution of molecular weight including a "tail" of a very low molecular weight material comprising 10 to 25 percent by weight.
The process provides that a solution comprising 2.5 to 19 weight percent polybutadiene in a mixture of acrylonitrile and styrene monomers be added to the first stage reactor which already contains a styrene homopolymer or acrylonitrile/styrene copolymer. Combining the two solutions causes the rubber to precipitate while the agitation within the vessel disperses the rubber before significant crosslinking can occur. Viscosity is continuously reduced and polymerization increases in each of the three reactors. The patent states that particle size of the rubber in the product is generally fixed in the second stage reactor and comprises at least about 80 percent of the particles falling within the range of 0.5 to 2.0 microns and less than about one percent being larger than six microns. The patent further states that by controlling the microgel particle size of the rubber and by providing the low molecular weight tail, physical properties and surface appearance of the product are improved.
Thus, the patents discussed herein have not disclosed a simple one stage polymerization, as can be conducted in a reactor extruder, where rubber-modified plastics can be prepared having a uniformly distributed rubber of uniform small particle size, on the order of less than 0.25 micron, in the plastic phase. While this object can be met to an extent according to the process set forth in the aforementioned Ser. No. 309,718, the present invention differs in that higher strength thermoplastics can be obtained resulting from the feeding and chemical precipitating techniques disclosed herein.